1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus.
2. Description of the Related Art
In current optical scanning devices and image forming apparatuses using such optical scanning devices, high quality images, full color images, high image forming speeds, and cost reductions in apparatuses have been demanded. To accomplish high quality images, curvature of scanning lines and variation in beam spot diameter on a scanned surface needs to be reduced. Moreover, position shifts of beam spots due to improvement in constant speed characteristic needs to be reduced.
To accomplish high image forming speed, optical scanning devices that can scan a scanned surface with a plurality of light beams at the same time need to be realized. In recent years, multibeam image forming apparatuses that can perform scanning with a plurality of scanning lines at a time have appeared. According to such image forming apparatuses, light sources irradiating light beams are spaced apart from each other by a distance in a sub-scanning direction. The configuration of the curvature then varies from one scanning line to another and the image quality is thus deteriorated.
Particularly, in full color image forming apparatuses that scanned surfaces for a plurality of colors are scanned by respective scanning optical systems and toner images corresponding to the respective colors are superimposed, for example, in tandem image forming apparatuses, the image quality can be deteriorated because of color shifts in the sub-scanning direction and differences in shading.
A vertical resonator surface emitting laser (hereinafter, VCSEL) that a plurality of emission points irradiating light beams are formed easily on one element is particularly effective for multibeam. As the number of beams is increased, however, a region where emission points are arranged becomes large around the optical axis and the aforementioned problems become more serious. Specifically, the position on an optical element through which the light beam passes is different from channel to channel because of a large number of the emission points and the difference in optical characteristics between the channels becomes large. A beam spot diameter can be increased in a main scanning direction or sub-scanning direction. A scanning pitch can become uneven and a beam pitch in the sub-scanning direction can be increased.
Although excellent images are provided during ordinary use, because of environmental changes including changes in room temperature and temperature rises within apparatuses caused by continuous prints, the arrangement and configuration of optical elements in such apparatus can be changed slightly, the path of light beam is varied, and a sub-scanning direction distance between the light beams is varied. As a result, the color shift occurs in the resulting output image. Such color shift caused by the environmental changes is serious in image forming apparatuses with advanced high density and high image quality, particularly in color image forming apparatuses and measures needs to be taken. To solve these problems, techniques described in, for example, Japanese Patent Application Laid-open No. 2004-287292 and Japanese Patent Application Laid-open No. 2005-309301 have been proposed.
According to an image forming apparatus using VCSEL described in Japanese Patent Application Laid-open No. 2004-287292, a distance between light sources spaced farthest apart from each other in the main scanning direction is smaller than a distance between light sources spaced farthest apart from each other in the sub-scanning direction. All conditions including cost reduction in the apparatus, significantly high density image, and ensuring of light amount of light beam are thus difficult to be satisfied.
According to an image forming apparatus described in Japanese Patent Application Laid-open No. 2005-309301, in a multibeam scanning optical system, skew and sub-scanning direction angle are adjusted for a plurality of cylindrical mirrors, so that the curvature of scanning lines is adjusted. High quality images are thus obtained. Because cylindrical mirrors are expensive, however, the cost reduction of the apparatus is limited.
Furthermore, the above patent documents do not describe any appropriate distance between emission points on a surface emitting laser light source that irradiates a plurality of light beams from two-dimensionally arranged emission points and the conditions of the scanning optical system for the surface emitting laser. Means for solving the problems such as a decrease in scanning speed caused by the lack of light amount during scanning, increased cost because expensive optical elements is utilized, and an increase in the beam spot diameter are not described. High quality images, increased image forming speeds, and cost reductions in apparatuses have not been realized.
Japanese Patent Application Laid-open No. H11-340570 discloses a photoelectric conversion element that includes a plurality of photoelectric conversion units on the same substrate. The photoelectric conversion unit is formed of a plurality of semiconductor layers laminated and its side surface is at the end portion in a direction vertical to the laminated direction.
Japanese Patent Application Laid-open No. H11-354888 discloses a semiconductor emission element including a base, a substrate supported by the base, a plurality of light emitting units formed of a plurality of semiconductor layers laminated on the side of the substrate opposite the side at which the base is placed, a plurality of ohmic electrodes that are provided on the side of the light emitting units opposite the side at which the substrate is provided and electrically connected to the respective light emitting units, and at least one radiation layer that is laminated via the light emitting units on the substrate to be formed at the side of the light emitting units opposite the side at which the substrate is placed.
However, the photoelectric conversion element disclosed in Japanese Patent Application Laid-open No. H11-340570 and the semiconductor emission element disclosed in Japanese Patent Application Laid-open No. H11-354888 have large consumption power and amount of heat generated, so that they cannot be utilized in multibeam light sources. In view of costs, they can be utilized in four- or eight-beam light sources at most. According to these elements, a plurality of light emitting units are arranged one-dimensionally. If the number of beams is substantially increased, the amount of beam shift from the optical axis of the optical system becomes large, so that beam characteristics can be deteriorated.
Meanwhile, a vertical resonator surface emitting laser (VCSEL: Vertical Cavity Surface Emitting Laser) that can form a plurality of emission points two-dimensionally on a device and has small consumption power has been proposed.
Japanese Patent Application Laid-open No. 2004-287292 discloses an optical scanning device that includes a first substrate to which the light emitting element and its drive circuit are mounted and which is incorporated in a housing fixed to the image forming apparatus main unit, a second substrate to which the connector for connecting to the harness from the image forming apparatus main unit is mounted and which is incorporated in the housing to be apart from the first substrate, and a connecting member for electrically connecting a terminal of the first substrate to the terminal of the second substrate.
Japanese Patent Application Laid-open No. 2005-250319 discloses a light source apparatus including a plurality of combinations of the light source obtained by arranging a plurality of independently modulated emission points two-dimensionally and the coupling lens that couples divergent light irradiated from the light source.
Japanese Patent Application Laid-open No. 2005-309301 discloses an optical scanning device that includes a light source for irradiating light beam, a deflector for deflecting the light beam in the main scanning direction, an optical system for imaging the deflected light beam on a scanned surface, and a plurality of cylindrical mirrors placed on the optical path between the deflector and the scanned surface. The optical scanning device includes a skew adjustment unit that rotates a cylindrical mirror adjacent to the scanned surface about the normal line of the reflecting surface to adjust the skew of scanning lines and an angle adjustment unit that rotates the cylindrical mirror adjacent to the deflector about the generatrix of the reflecting surface to adjust the sub-scanning direction angle.
The image forming apparatus has recently been used for simplified printing as an on-demand printing system. Accordingly, higher image qualities have been demanded without increasing its price.